Enhancing the Physical Properties of Semi-Crystalline Polymers via Solid-State Shear Pulverization

ABSTRACT

Solid-state shear pulverization of semi-crystalline polymers and copolymers thereof and related methods for enhanced crystallization kinetics and physical/mechanical properties.

This application is a divisional of and claims priority to and thebenefit of application Ser. No. 14/279,521 filed May 16, 2014 and issuedas U.S. Pat. No. 9,133,311 on Sep. 15, 2015, which was a divisional ofand claimed priority to and the benefit of application Ser. No.13/854,558 filed Apr. 1, 2013 and issued as U.S. Pat. No. 8,729,223 onMay 20, 2014, which was a divisional of and claimed priority to and thebenefit of application Ser. No. 12/322,396 filed Feb. 2, 2009 and issuedas U.S. Pat. No. 8,410,245 on Apr. 2, 2013, which claimed prioritybenefit to and the benefit of application Ser. No. 61/063,036 filed Jan.31, 2008—each of which is incorporated herein by reference in itsentirety.

This invention was made with government support under DMR-0076097 andDMR-0520513 awarded by the National Science Foundation. The governmenthas certain rights in the invention.

BACKGROUND OF THE INVENTION

Solid-state processing methods, such as ball milling, mechanicalalloying, pan milling, and solid-state shear pulverization, haverecently been employed to achieve intimate mixing of blends andnanocomposites, leading in some cases to materials that cannot beproduced via conventional processing methods. Vaia and co-workersdemonstrated, through the use of cryo-compounding, the ability to reducethe tactoid/agglomerate size of organically modified montmorillonite inepoxy-organoclay nanocomposites. (See Koerner, H.; Misra, D.; Tan, A.;Drummy, L.; Mirau, P.; Vaia, R Polymer, 2006, 47, 3426-3435.) Smith etal. produced blends where the dispersed phase in poly(methylmethacrylate) and polyisoprene or poly(ethylene-alt-propylene) were onthe order of nanometers with the use of cryogenic mechanical alloying.(See Smith, A. P.; Ade, H.; Balik, C. M.; Koch, C. C.; Smith, S. D.;Spontak, R. J. Macromolecules 2000, 33, 2595-2604.) In contrast to thesebatch processes, solid-state shear pulverization (SSSP), a continuous,industrially applicable process, has resulted in intimate mixing andexcellent dispersion in immiscible polymer blends, in some casesyielding 100-200 nm dispensed-phase domain diameters in a polymermatrix. (See, e.g., N. Furgiuele, A. H. Lebovitz, K. Khait, and J. M.Torkelson, Polym. Eng. Sci., 40, 1447 (2000); N. Furgiuele, A. H.Lebovitz, K. Khait, and J. M. Torkelson, Macromolecules, 33, 225 (2000);A. H. Lebovitz, K. Khait, and J. M. Torkelson, Macromolecules, 35, 8672(2002); A. H. Lebovitz, K. Khait, and J. M. Torkelson, Macromolecules,35, 9716 (2002); A. H. Lebovitz, K. Khait, and J. M. Torkelson, Polymer,44, 199 (2003); Y. Tao, A. H. Lebovitz, and J. M. Torkelson, Polymer,46, 4753 (2005); Y. Tao, J. Kim, and J. M. Torkelson, Polymer, 47, 6773(2006); A. M. Walker, Y. Tao, and J. M. Torkelson, Polymer; 48, 1066(2007); each of which is incorporated herein by reference.)

While much work involving solid-state processing has focused onheterogeneous systems and mixtures, relatively little has been done onhomopolymers. Zhu et al. demonstrated that cryomilling poly(ethyleneterephthalate) (PET) results in the amorphization of PET, which leads todeleterious effects on PET physical properties. (See Zhu, Y. G.; Li, Z.Q.; Zhang, D.; Tanimoto, T. J Appl Polym Sci 2006, 99, 2868-2873.)Similar adverse effects were observed for polypropylene (PP) as a resultof a decrease in the molecular weight and degree of crystallinity of PPduring cryomilling. During a study on the effect of different panmilling processing conditions on the particle size of polystyrene (PS),Wang et al. observed that pan milling PS degrades the polymer. (SeeWang, Q.; Cao, J. Z.; Huang, J. G.; Xu, X. Polym Eng Sci 1997, 37,1091-1101.) Ganglani et al. proved that although chain scission andradical formation may occur during SSSP processing, significant short orlong chain branching does not occur in polyolefins. (See Ganglani, M.;Torkelson, J. M.; Carr, S. H., Khait, K. J Appl Polym Sci 2001, 80,671-679). Such results suggest SSSP does not alter the macroscopicstructure of polymers such as (high density polyethylene) (HDPE) orlinear low density polyethylene (LLDPE), i.e., they are not converted tolow density polyethylene (LDPE).

Notwithstanding previous efforts undertaken in the context of polymerblends and various batch processes, there is an on-going search in theart to provide efficient, effective solid-state processing of singlecomponent polymer systems with attendant enhancement of variousperformance parameters.

SUMMARY OF THE INVENTION

In light of the foregoing, it is an object of the present invention toprovide methods of using solid-state shear pulverization andcorresponding pulverization products, heretofore unrealized in the art.It would be understood by those skilled in the art that one or moreaspects of this invention can meet certain objectives, while one or moreother aspects can meet certain other objectives. Each objective may notapply equally, in all its respects, to every aspect of this invention.As such, the following objects can be viewed in the alternative withrespect to any one aspect of this invention.

It can be an object of the present invention to use solid-state shearpulverization techniques with single component polymer systems toenhance one or more polymer physical or mechanical properties, suchproperties including but not limited to Young's modulus, gas barrier,melt viscosity and melt strength properties.

It can be another object of this invention to provide varioussolid-state shear pulverization-related methods for the preparationand/or modification of a range of semi-crystalline polymer materials.

It can also be an object of the present invention, alone or inconjunction with one or more of the preceding objectives, to provide arange of semi-crystalline homopolymers or crystallizable copolymersthereof characterized by enhanced crystallization kinetics and/orpolymer morphology-such polymers prepared absent the presence ofnucleating agent or filler components typically associated with theprior art.

Other objects, features, benefits and advantages of the presentinvention will be apparent from this summary and the followingdescriptions of certain embodiments, and will be readily apparent tothose skilled in the art having knowledge of various polymer physicaland mechanical properties and solid-state shear pulverizationtechniques. Such objects, features, benefits and advantages will beapparent from the above as taken into conjunction with the accompanyingexamples, data, and all reasonable inferences to be drawn therefrom,alone or with consideration of the references incorporated herein.

In part, the present invention can be directed to a method of affectingcrystallization kinetics of a semi-crystalline homopolymer or acrystallizable copolymer thereof. Such a method can comprise providingsuch a polymer comprising less than about 50% crystallinity and as canbe substantially absent nucleating agent and/or filler components;applying a mechanical energy thereto through solid-state shearpulverization in the presence of an element of cooling at leastpartially sufficient to maintain such a polymer in a solid state, suchpulverization at least partially sufficient to induce polymer scissionand/or nucleation sites within or indigenous to such a polymer. In thecontext of either a nucleating agent or a filler component, the phrase“substantially absent” can be considered with reference tocrystallization kinetics, mechanical properties and/or correspondingpolymer physical properties or morphologies of the sort describedherein, in conjunction with this invention, such kinetics or propertiesas can be realized without such nucleating agent or filler components,in trace or insignificant amounts or in amounts less than wouldotherwise be understood in the art as required to achieve such effects.

In certain embodiments, a crystallization kinetic effect can be selectedfrom increased onset crystallization temperature and/or reducedisothermal crystallization half-time. In certain non-limitingembodiments, such an effect can be realized with a homopolymer selectedfrom polyesters and polyolefins. In certain such embodiments, asillustrated below, such a homopolymer can be a polyester, andpulverization can induce scission thereof. In certain other embodiments,such a homopolymer can be a polyester or a polyolefin, and pulverizationcan increase nucleation sites therein. Regardless, without limitation asto kinetic effect or polymer identity, such a pulverized polymer can beincorporated into an article of manufacture.

In part, the present invention can be directed to a method of usingsolid-state shear pulverization to affect a physical property of asemi-crystalline homopolymer. Such a method can comprise providing asingle component comprising a solid semicrystalline homopolymer;introducing such a homopolymer into a solid-state shear pulverizationapparatus, such an apparatus as can comprise a cooling component atleast partially sufficient to maintain homopolymer solid state; shearpulverizing such a homopolymer, such pulverization at least partiallysufficient to affect a physical property such as but not limited tohomopolymer spherulite size and/or number, such a homopolymer andpulverization as can be substantially absent nucleating agent and/orfiller components; and discharging such a shear pulverized homopolymerfrom the apparatus.

In certain embodiments, pulverization can be used to affect Young'smodulus and/or gas permeability of a homopolymer film. In certainnon-limiting embodiments, such a homopolymer can be selected frompolyesters and polyolefins. Regardless, without limitation as tophysical property, mechanical property or polymer identity, such a shearpulverized homopolymer can be incorporated into an article ofmanufacture.

In part, the present invention can also be directed to asemi-crystalline polymer, such a polymer the solid-state shearpulverization product of a starting material of such a polymer orcrystallizable copolymer thereof, such a pulverization product as can besubstantially absent a nucleating agent and/or filler component. Such apulverization product can be characterized by an enhancedcrystallization kinetic property, such enhancement as can be compared tothe crystallization kinetic property of such a polymer startingmaterial.

In certain embodiments, such a characterized crystallization kineticproperty can be increased onset crystallization temperature and/orreduced isothermal crystallization half-time. Regardless, such an effectcan be characterized for a polymer starting material comprising lessthan about 50% crystallinity. Such semicrystalline polymer startingmaterials can be as described elsewhere herein or as would be understoodby those skilled in the art. Regardless, a pulverized homopolymer ofthis invention can be incorporated into a range of articles ofmanufacture.

The present invention, as illustrated below, can provide a route to newbehavior and enhanced properties for semi-crystalline polymers processedby SSSP without additives such as fillers or nucleating agents. Suchpolymers include, without limitation, PP, LDPE, LLDPE, HDPE,polycaprolactone (PCL), poly(butylene terephthalate) (PBT), and PET. Thecrystallization kinetics of such polymers can be enhanced withoutcompromising the degree of crystallinity of the polymer. In certaincases, both the crystallinity and the crystallization kinetics areenhanced. Representative of certain embodiments of this invention, forLDPE, PCL, PBT and PET, significant changes in the crystallizationkinetics can occur and enhancements can be observed in the mechanicaland barrier properties of such polymers. In some cases, the effectsobserved are comparable to those generally associated with the additionof nucleating agents or fillers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. Shear viscosity of pristine, unprocessed PET (▪) and pulverizedPET (□) at 280° C. and 100% strain.

FIGS. 2 A-B. Optical micrographs of A) pristine and B) pulverized PP.Size bar=25 μm.

FIG. 3. Young's Modulus of polymers before and after pulverization. Allsamples except for PCL were tested at a crosshead speed of 50 mm/min andPCL at was tested at 25 mm/min.

FIG. 4. Normalized oxygen permeability coefficients of polymer beforeand after pulverization measured at 23° C. and 0% relative humidity.

FIGS. 5 A-C. Proposed mechanism for the formation of branched PET duringSSSSP processing. A) Large stresses are concentrated at the weaker C—Obond in the ester linkage, this results in stress-induced scission ofthe bond. Two radicals form, after C—O bond is cleaved. B) The ethyleneradical extracts a hydrogen from a PET chain, which results in a radicalalong the PET backbone and a dead polymer chain. C) The radical alongthe PET backbone can recombine with another radical and form branchedPET.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

Representative of certain non-limiting embodiments of this invention,and as discussed more fully below, improved crystallization kinetics andphysical properties of semi-crystalline polymers, e.g., withoutlimitation, polyethylene, ε-polypropylene, polycaprolactone,poly(butylene terephthalate) and poly(ethylene terephthalate) can berealized. While certain data, observations and/or results may bedescribed in conjunction with one or particular mechanisms or phenomena,it will be understood by those skilled in the art that this invention isnot limited by anyone theory or mode of operation.

However, such results and/or enhancements can be obtained using SSSPprocessing without the addition of nucleating agents, using theprocessing conditions specified in Table 1. More generally,pulverization can be accomplished with an SSSP apparatus of the sortdescribed herein or as would otherwise be known in the art, suchapparatus, component construction, screw elements, transport elements,kneading or shearing elements, and spacer elements and/or the sequenceor design thereof selected or varied as required to accommodate apolymer starting material, pulverization parameters and/or pulverizedpolymer product. Such apparatus, components and operation are understoodby those skilled in the art and, for instance, can be of the sortdescribed more fully in U.S. Pat. Nos. 5,814,673; 6,180,685 and7,223,359—each of which is incorporated herein by reference in itsentirety.

TABLE 1 SSSP processing conditions Screw Speed Feed Rate Sample ScrewDesign (rpm) (g/hr) PP P209bs* 300 135 LDPE CP104^(#) 300 200 LLDEPCP104 300 150 HDPE P24lbs⁺ 200 190 PCL P209bs 300 30 PBT P209bs 250 195PET P209bs 250 185 *P209bs contains 2 forward kneading elements and 7shearing elements: 4 forward, 2 neutral, and 1 reverse and 12 conveyingelements ^(#)CP104 contains 4 kneading elements: 2 forward, 1 neutral,and 1 reverse and 7 shearing elements: 4 forward, 2 neutral, and 1reverse and 10 conveying elements ⁺P241bs contains 3 kneading elements:2 forward, 1 reverse and 7 shearing elements: 5 forward, 1 neutral, and1 reverse and 11 conveying elements

More specifically, but indicative of broader aspects of this invention,solid-state shear pulverization processing resulted in enhancedisothermal crystallization rates (as measured by reduced isothermalcrystallization half-times) of at least 27% for PP, 85% for LDPE and 92%for PBT. The Young's modulus of PCL increased by 65% afterpulverization, and comparable or lesser changes were observed in theother polymers. The oxygen permeability of films made from SSSPprocessed polymer relative to relative to the unprocessed polymerdecreased by 17% for LDPE, 30% for PCL and PBT, and 55% for PET. Suchenhancements in physical properties of polymer processed by SSSP can beattributed to changes in the level of crystallinity and/or changes inthe shape and size of the spherulites formed during crystallization.

Molecular Weight Characterization.

Table 2 provides the molecular weights of PP, LDPE, LLDPE, HDPE, PCL andPBT before and after pulverization as measured by gel permeationchromatography. For PP, LDPE, LLDPE, HDPE and PBT, both thenumber-average and weight-average molecular weights do not changesignificantly, suggesting that the polymer chains remain unaffected bySSSP processing. In the study conducted by Ganglani et al., it wasdemonstrated that molecular structural changes (MW reduction orbranching) did not occur to any significant extent when LDPE, LLDPE, andHDPE were processed using low and high shear SSSP processing conditions.Given that the molecular weight distributions of PP, LDPE, LLDPE, andHDPE in the present study were not appreciably altered by SSSPprocessing, it can be considered that chain scission or branching didnot occur in any of these polymers as well.

TABLE 2 Molecular weight averages of polymers before and after SSSPprocessing M_(n) ^(a) × 10³ M_(w) ^(b) × 10³ Sample (g/mol) (g/mol)PDT^(c) PP 169 469 2.77 Pulverized PP 176 466 2.66 LDPE* 205 282 1.3723.1 43 1.85 Pulverized LDPE* 196 251 1.28 25.3 45 1.78 LLDPE 60 1482.47 Pulverized LLDPE 62 135 2.17 HDPE 77 360 4.68 Pulverized HDPE 76335 4.42 PCL 63.6 108 1.70 Pulverized PCL 23.4 49 2.09 PBT 42 97 2.30Pulverized PBT 44 91 2.07 *LDPE possessed a bimodal molecular weightdistribution ^(a)Number-average molecular weight ^(b)Weight-averagemolecular weight ^(c)Polydispersity

In contrast, PCL undergoes significant chain scission during SSSPprocessing, which is evident by the nearly factor of 3 reduction in itsnumber-average molecular weight and a factor of 2 reduction in itsweight-average molecular weight. As with polystyrene that has beenprocessed by SSSP, the chain scission is most likely caused by thefragmentation and fracture of PCL chains during SSSP processing. (Themolecular weight reduction in PCL is believed not associated withhydrolysis of PCL during SSSP processing. This is because the SSSPprocessing occurs at relatively low temperatures where PCL is in itssemi-crystalline state and where hydrolysis of PCL does not occur.)Furgiuele et al. demonstrated that the degree of chain scission that PSand PP undergo can be controlled by the type of screw employed and theinitial molecular weight of the polymer. In order to reduce the level ofchain scission, a milder screw design can be employed for thepulverization of PCL.

The molecular structure of PET before and after SSSP processing wasprobed through a combination of intrinsic viscosity and melt rheologycharacterization. The increase in melt viscosity as shown in FIG. 1after SSSP processing without a related increase in intrinsic viscosityindicates that PET becomes lightly branched during SSSP processing. Assuch, an accurate characterization of PET molecular weight is notpossible, but—as discussed below—the PET molecular structure changesfrom a linear chain before SSSP processing to a lightly branched chainafter SSSP processing.

Crystallization Kinetics.

As shown in Table 3 and 4, pulverization significantly alters thecrystallization behaviors of LDPE, PCL, PBT and PET, while those of PP,LLDPE, and HDPE remain the same within experimental error. After SSSPprocessing, the onset crystallization temperature (measured upon coolingfrom the melt state), T_(c,onset), increased by 6° C. for both LDPE andPCL, 14° C. for PET and 15° C. for PBT. Comparable changes inT_(c,onset) have been reported upon addition of certain types of fillersto LDPE and upon addition of 4 vol % clay to PCL. (See Chen, B. Q.;Evans, J. R. G. Macromolecules 2006, 39, 747-754.) In the latter case,the increase in T_(c,onset) was attributed by Chen and Evans to the clayplatelets acting as nucleating agents, thereby increasing T_(c,onset).

TABLE 3 Crystallization kinetics of pristine and pulverized polymers viaonset crystallization from the melt state Crystallization Onset SampleTemperature (° C.) Percent Crystallinity PP 119 44 Pulverized PP 121 46LDPE 88 23 Pulverized LDPE 94 22 LLDPE 116 39 Pulverized LLDPE 118 42HDPE 123 78 Pulverized HDPE 123 77 PCL 31 47 Pulverized PCL 37 46 BPT185 32 Pulverized BPT 200 32 PET 195 13 Pulverized PET 209 32

The isothermal crystallization half times of the pristine and pulverizedpolymers are provided in Table 4. It is clear that pulverizationsignificantly accelerates the isothermal crystallization and therebyreduces the values of pulverized LDPE, LLDPE, PCL, PBT and PET, withnotable impact on LDPE (85% reduction of τ_(c) ½), PCL (71% reduction),PBT (92% reduction) and PET (76% reduction). For semi-crystallinepolymers that are highly crystalline (i.e., greater than about 50%crystallinity) and/or readily crystallizable, as in the case of HDPE,pulverization appears to not significantly affect crystallizationbehavior. In other words, as the crystallizability of the PE increases,the effect of SSSP processing decreases. This provides an explanationfor the substantial changes caused by SSSP in the crystallizationkinetics of LDPE, while LLDPE and HDPE have mild to null effects.

TABLE 4 Isothermal crystallization half times of pristine and pulverizedpolymers Isothermal Crystallization Half Temperature Time Percent Sample(° C.) (min) Reduction PP 130 11 — Pulverized PP 130 8 27 LDPE 95 13 —Pulverized LDPE 95 2 85 LLDPE 120 4 — Pulverized LLDPE 120 2 50 HDPE 1264 — Pulverized HDPE 126 4  0 PCL 40 7 — Pulverized PCL 40 2 71 BPT 20539 — Pulverized BPT 205 3 92 PET 215 17 — Pulverized PET 215 4 76

Optical Microscopy.

FIG. 2 shows the spherulites of PP and pulverized PP followingisothermal crystallization at 130° C. The size of PP spherulites isobserved to be significantly reduced in films made from pulverized PPrelative to films made from pristine PP. Related changes can occur inother polymers.

Mechanical Properties.

FIG. 3 compares the Young's modulus of the pulverized polymers relativeto the pristine polymers; the Young's moduli of PP, LDPE, LLDPE, andHDPE remain the same within experimental error after pulverization.Similar results for these polymers were obtained regarding the effect ofSSSP on tensile strength and elongation at break. In contrast, for PCLthe Young's modulus increased by 65% and the tensile strength by 31%after pulverization. Comparable changes in modulus and tensile strengthwere observed in PBT and PET after SSSP processing. An approximately 30%decrease in elongation at break was observed after SSSP processing ofPCL, which can be attributed to the large reduction in PCL molecularweight accompanying pulverization. Large reductions in elongation atbreak were measured in PET after SSSP processing, which is due to theenhancement in crystallinity.

The observed improvements in the Young's modulus and tensile of PCLafter SSSP processing are greater than results that have been reportedin PCL-organoclay nanocomposites. Pantoustier et al. preparednanocomposites of PCL-organoclay through melt-intercalation anddemonstrated a 30% increase in the Young's modulus with the addition of3 wt % organoclay. (See Pantoustier, N.; Lepoittevin, B.; Alexandre, M.;Kubies, D.; Calberg, C.; Jerome, R.; Dubois, P. Polym Eng Sci 2002, 42,1928-1937.) A 30% decrease in both the strength and elongation at breakof PCL accompanied this increase in the Young's modulus. When acomparable enhancement in the Young's modulus was demonstrated by Di etal. using 5 wt % organoclay, only a 20% increase in the tensile strengthof PCL was achieved. The elongation at break of PCL exhibited a 20%reduction. (See Di, Y. W.; Iannac, S.; Sanguigno, L.; Nicolais, L.Macromol Symp 2005, 228, 115-124.)

Oxygen Permeability.

The effect of pulverization on the barrier properties of the polymer wasinvestigated using oxygen as the permeant; the results are depicted inFIG. 4. Processing by SSSP leads to reductions in oxygen permeability of17%, 30%, 30% and 55% for LDPE, PCL, PBT and PET films, respectively.Within experimental error, there was a null effect on the barrierproperties of PP, LLDPE, and HDPE. The substantial reduction in theoxygen permeability of PET is correlated with the increase in thepercent crystallinity of films made from pulverized PET relative tofilms made from PET not processed by pulverization. Related improvementsin the barrier properties of a polymer are known to be achieved with theaddition of appropriately dispersed filler. This work indicates that achange in the crystallization kinetics of polymer lacking filler canlead to a decrease in the permeability of the diffusing gas.

Examples of the Invention

The following non-limiting examples and data illustrate various aspectsand features relating to the polymers and/or methods of the presentinvention, including the preparation of semi-crystalline polymers assingle component systems, as are available through the processing andsynthetic methodologies described herein. In comparison with the priorart, the present methods and polymers provide results and data which aresurprising, unexpected and contrary thereto. While the utility of thisinvention is illustrated through the use of several polymers, it will beunderstood by those skilled in the art that comparable results areobtainable with various other polymers and single component systems, asare commensurate with the scope of this invention.

Materials.

Polymer starting materials were obtained commercially: PP from Atofinawith a melt flow index (MFI) of 18 g/10 min, LDPE from Exxon Mobil witha MFI of 70 g/10 min, LLDPE from Equistar Chemicals with a MFI of 3 50g/min (as determined by a constant pressure capillary flow device), HDPEfrom Equistar Chemicals with a MFI of 0.8 g/10 min, PCL from Aldrichwith a MFI of 1.00 g/10 min, PBT from Aldrich with a reported viscosityaverage molecular weight of 38,000 g/mol, and PET with an intrinsicviscosity of 0.755 dL/g from Eastman Chemicals.

Example 1 SSSP Processing of PET

All polymers were processed using the SSSP apparatus under variousprocessing conditions specified in Table 1. The SSSP apparatus is acommercially available modified Berstorff ZE-25 intermeshing twin-screwextruder equipped with a cooling system set at −7° C. The cooling systemallows the polymer to be maintained in the solid state duringprocessing. The diameter of the screw is 25 mm with an aspect ratio,L/D, of 26. A detailed description of the SSSP apparatus can be foundthe literature (e.g., see Torkelson, Polym. Eng. Sci. 40, 1447 (2000),the entirety of which is incorporated herein by reference).

In order to probe changes in the molecular structure of PET, therheological properties of pristine and pulverized PET samples weremeasured using a Rheometrics Scientific ARES strain-controlled rheometerwith a 5 cm parallel plate geometry. The samples were subjected tofrequency sweeps from 0.1 to 10 Hz at 100% strain. As shown in FIG. 1,an increase in melt viscosity of PET after pulverization was observed.

The increase in melt viscosity of PET after pulverization can resultfrom either an increase in average linear chain molecular weight or inthe production of a branched molecular structure accompanyingpulverization. Intrinsic viscosity measurements were done on PET samplesbefore and after pulverization, which revealed a slight decrease inintrinsic viscosity accompanying pulverization. For example, for PETpulverized at a screw speed of 250 rpm, the intrinsic viscosity changedfrom 0.755 dl/g before SSSP to 0.715 dl/g after SSSP. If SSSP resultedin an increase in linear chain molecular weight, this would result in anincrease in intrinsic viscosity. A small level of chain scission duringSSSP processing (with accompanying polymeric radical formation andreactions) leading to a lightly branched PET molecular structure isconsistent with both the melt viscosity and intrinsic viscositymeasurements.

FIG. 5 provides a possible mechanism for the achievement of lightlybranched PET after SSSP processing. The application of high levels ofshear and 13 compressive forces (mechanical action) can lead to lowlevels of scission of the weakest bonds in PET, those being C—O backbonebonds. The resulting radicals can participate in hydrogen extractionreactions, leading to the presence of radicals on the backbone of a PETchain. This radical can recombine with another PET radical (likely witha radical located at a chain end), thereby resulting in branched PET.Thus, in situ mechanochemistry accompanying simple SSSP processing oflinear PET chains can lead to a lightly branched PET product.

The tensile properties of samples ranging from 0.25 to 0.50 mm inthickness with a cross sectional area of 5 mm by 20 mm were determinedusing a Sintech 20/G according to ASTM D1708, as known in the art, theentirety of which is incorporated herein by reference. After preparationby compression molding, samples were allowed to equilibrate at roomtemperature for 2 days prior to testing. All PET samples were tested atroom temperature and at a crosshead speed of 50 mm/min. Due to thehigher crystallinity level, films made from pulverized PET possess ahigher Young's Modulus than films made from pristine PET.

The crystallization behavior was characterized using a Mettler-ToledoDSC 822e with samples weighing 3 to 10 mg. Measurements were taken onthe second cycle of heating and cooling at a rate of 10° C./min. Forisothermal crystallization characterization, samples were heated abovethe melt temperature, T_(m), and then quenched to the respectiveisothermal crystallization temperature, where the samples were allowedto crystallize for at least 30 min. These results are provided in Tables3 and 4. After pulverization and as measured by DSC, PET exhibits afactor of 2.5 increase in percent crystallinity. Hanley et al.demonstrated that the presence of branching in PET increases thecrystallization rate and can increase crystallinity. (See T. Hanley, D.Sutton, E. Heeley, G. Moad and R. B. Knott, J. Appl. Crystallogr., 2007,40, s599. And T. L. Hanley, J. S. Forsythe, D. Sutton, G. Moad, R. P.Burford and R. B. Knott, Polym. Int., 2006, 55, 1435.) Thus, theincrease in the percent crystallinity of PET after SSSP processing isconsistent with a conclusion from the rheology data that branching ofPET occurs during pulverization.

In order to characterize oxygen barrier properties, compression moldingwas used to obtain polymer films that were approximately 0.25 mm thick.An aluminum mask with a permeable area of 5.0 cm² was used for theanalysis of oxygen permeation of the pristine and pulverized films usinga MOCON OX-Tran model 2-21MH. Films were measured at 23° C. and 0%relative humidity and conditioned for 1 hour prior to testing. At leastfour samples were tested for each polymer. As shown in FIG. 4, oxygenpermeability decreases by more than 50% in films made from pulverizedPET relative to films made from pristine PET. This major improvement inoxygen barrier properties arises from the increase in crystallinity ofPET after SSSP processing (pristine PET is 13% crystalline whilepulverized PET is 32% crystalline). The crystalline regions areimpermeable to and create a torturous path for the diffusing gas orliquid. The increase in path length that the gas must travel results ina decrease in permeability of gases or liquids in the material.

Example 2 SSSP Processing of PBT

All polymers were processed using the SSSP apparatus under variousprocessing conditions specified in Table 1. The SSSP apparatus is acommercially available modified Berstorff ZE-25 intermeshing twin-screwextruder equipped with a cooling system set at −7° C. The cooling systemallows the polymer to be maintained in the solid state duringprocessing. The diameter of the screw is 25 mm with an aspect ratio,L/D, of 26. A detailed description of the SSSP apparatus can be foundthe literature (e.g., see Torkelson, Polym. Eng. Sci. 40, 1447 (2000),the entirety of which is incorporated herein by reference).

In contrast to both PET and PCL, the molecular weight of PBT is notsignificantly affected by SSSP processing. The PBT samples were allprepped for GPC by weighing 0.08-0.09 g of polymer and dissolving in 2mL of hexfluroisopropanol. Once dissolved, 0.5 mL of that solution wasadded to 10 mL of chloroform. The chloroform solution was filteredthrough 0.2 μm syringe filter. The samples were analyzed by refractiveindex on an Agilent 1100 series HPLC system at 35° C.

After pulverization of PBT, the onset temperature of crystallizationfrom the melt state increases by 15° C. and the isothermalcrystallization halftime at 205° C. is reduced from 39 min to 3 min.These results are consistent with either achievement of 15 very lowlevels of chain branching that cannot be distinguished via GPC and/orthe effective dispersal and increase in number of natural heterogeneousnucleating sites present in the as-received PBT.

After pulverization, the oxygen permeability of films made from PBTdecreases by 30% while the Young's modulus increases by 21%.

Example 3 SSSP Processing of PCL

All polymers were processed using the SSSP apparatus under variousprocessing conditions specified in Table 1. The SSSP apparatus is acommercially-available modified Berstorff ZE-25 intermeshing twin-screwextruder equipped with a cooling system set at −7° C. The cooling systemallows the polymer to be maintained in the solid state duringprocessing. The diameter of the screw is 25 mm with an aspect ratio,L/D, of 26. A detailed description of the SSSP apparatus can be foundthe literature (e.g., see Torkelson, Polym. Eng. Sci. 40, 1447 (2000),the entirety of which is incorporated herein by reference).

A Waters Breeze Instrument GPC with tetrahydrofuran as the eluent at 30°C. was used to determine M_(n), M_(w), and PDI of PCL. The GPC wasequipped with a refractive index (RI) detector (Waters 2410 differentialrefractometer). The data were calibrated relative to polystyrenestandards. PCL undergoes significant chain scission during SSSPprocessing, which is evident by the nearly factor of 3 reduction in itsnumber-average molecular weight and a factor of 2 reduction in itsweight-average molecular weight. In the case of pulverized PCL, thislarge reduction in molecular weight leads to enhanced crystallizationkinetics and to effective dispersal and increase in the density ofnatural heterogeneous nucleation sites in the as-received PCL, both ofwhich can enhance the mechanical and barrier properties.

After pulverization, the oxygen permeability of films made from PCLdecreases by 30% while the Young's modulus increases by 65%.

Example 4 SSSP Processing of Polyolefins

All polymers were processed using the SSSP apparatus under variousprocessing conditions specified in Table 1. The SSSP apparatus is acommercially available modified Berstorff ZE-25 intermeshing twin-screwextruder equipped with a cooling system set at −7° C. The cooling systemallows the polymer to be maintained in the solid state duringprocessing. The diameter of the screw is 25 mm with an aspect ratio,L/D, of 26. A detailed description of the SSSP apparatus can be foundthe literature (e.g., see Torkelson, Polym. Eng. Sci. 40, 1447 (2000),the entirety of which is incorporated herein by reference).

High temperature GPC was employed to determine the number-average andweight-average molecular weights (M_(n) and M_(w)) and polydispersityindices, PDI, (M_(w)/M_(n)) of PP, LDPE, LLDPE, and HDPE. The apparatuswas a Waters Alliance GPCV 2000 GPC equipped with a Waters DRI detectorand viscometer. 1,2,4-trichlorobenzene containing 0.01 wt. %di-tertbutylhydroxytoluene (BHI) was used to elute the column set, whichconsists of four Waters HI 6E and one Waters HT 2, at 1.0 mL/min at 140°C. Monomodal polyethylene standards from Polymer Standards Service wereused to calibrate the data. As shown in Table 2, within experimentalerror, there is not a change in the molecular weight of these polymers,indicating that the polymers do not undergo a change in their moleculararchitecture or polymer chain length.

Through sections of PP ranging from 1 to 4 μm obtained using a LeicaUltracut S RMC MI-6000 Ultramicrotome, images of the crystal structureof PP were obtained using a Nikon OPTIPHOT2-POL microscope equipped witha Mettler FP82 hot stage. The samples were heated to 200° C. and thenquenched to 130° C., where the samples of PP were allowed to crystallizeisothermally. Images were captured every 10 s and the final imagesobtained are shown in FIG. 2. The size of the spherulites of PP issignificantly reduced after pulverization. Given that molecular weightsand the mechanical and barrier properties of these polymers are notaffected, the enhancement in the crystallization kinetics of PP suggeststhat during SSSP processing the heterogeneous nucleating agents that arepresent in the neat, unprocessed polymer are broken down into smallerparticles and further distributed throughout the polymer matrix. Thisincreases the nucleation density of PP. The increase in nucleation sitescauses more spherulites to begin growing and eventually they impinge thegrowth of one another, resulting in smaller spherulites afterpulverization. Related but lesser effects occur in LLDPE.

As demonstrated, solid-state shear pulverization processing ofsemi-crystalline polymers can result in the enhancement of thecrystallization behavior of the polymer. These enhancements can beaccompanied by dramatic increases in the physical properties of thepolymer such as increased melt viscosity, modulus and lower gaspermeability. These results demonstrate the use of a solid-stateprocessing method can lead to changes in the structure of a polymer andresult in significant property enhancements for some polymers.

We claim:
 1. A semi-crystalline polymer, said polymer the solid-stateshear pulverization product of a starting material of said polymer, saidpolymer pulverization product substantially absent nucleating agent andfiller components, said polymer pulverization product characterized byan enhanced crystallization kinetic property, said enhancement ascompared to said crystallization kinetic property of said homopolymerstarting material, said polymer a single homopolymer selected frompolycaprolactone, poly(butylene terephthalate) and poly(ethyleneterephthalate).